Positron emission tomography (PET) is a nuclear medicine technique which allows us to obtain some anatomic information (e.g., axial, coronal or sagittal images of the brain) as well as dynamic functional data (such as regional cerebral glucose consumption rate). The unique property of PET is that it provides physiologic and pathophysiologic information unavailable with any other imaging procedure. Using the radio- pharmaceutical 18F-deoxyglucose as tracers, we continue to investigate brain tumors. Recently we completed a study on the effects of steroids on cerebral glucose utilization in patients with brain tumors and found that they reduce the global rate of cerebral glucose consumption. Also, in patients with Cushing's syndrome related to ACTH-producing pituitary adenomas, there is a significant reduction of the cerebral glucose consumption. It appears, that after removal of the adenoma, cerebral glucose metabolism returns to normal.